1. Field of the Invention
This invention is directed to apparatus for detecting and indicating loads or forces on tubular strings in wells or boreholes, and, in one aspect, to such apparatus for well tubing supported by an injector head that moves the tubing into and out of a well.
2. Description of Related Art
The prior art discloses a variety of load cells and indicators, several of which require the use of dual pistons and dual pressure gauges and seal rings that surround circular shaped parts. For example, U.S. Pat. No. 4,793,410 (commonly owned with the present invention) discloses a load cell which has two pressure chambers with a movable piston mounted in each chamber. One piston moves in response to downward force on the indicator and one piston moves in response to upward force on the indicator. A channel is provided in communication with each chamber to which a pressure or load indicating gauge may be attached. Seal rings are used with each piston to seal off the chambers so that accurate measurements are taken. The pistons must be very well aligned with the interior chamber walls so that they do not bind the pistons causing the cell to lock up. An injector head disposed on the cell by securing it to a top plate of the cell must be fairly well aligned with the cell's bore so that this type of misalignment will not occur.
U.S. Pat. Nos. 3,182,877 and 3,285,485 (both commonly owned With the present invention) also disclose two-piston, two-chamber, two-gauge sealed-piston load indicators which must be fairly precisely aligned with an injector head or other apparatus disposed above them.
The following U.S. patents disclose typical well tubing injector and hanger assemblies: U.S. Pat. Nos. 3,658,270; 3,675,719; 3,690,136; and 3,690,381--all commonly owned with the present invention; and U.S. Pat. Nos. 3,722,594 and 3,313,346. "Bowen Coiled Tubing Systems," Bowen Tools, Inc. shows a tubing injector head mounted above a load cell (page 2).
There has long been a need for a load cell with increased accuracy. There has long been a need for a load cell which does not malfunction due to the deposit of debris therein. There has long been a need for a load cell which can accurately indicate relatively small changes in load. There has long been a need for a cell which can function adequately despite some apparatus misalignment. There has long been a need for a load cell which can indicate a load independent of the weight of an apparatus (e.g. an injector head) secured to or mounted on the indicator. There has long been a need for a load cell with no "dead band," no hysteresis because of tare load, i.e. weight on the indicator (e.g. an injector head) that is not to be measured.